The present invention relates to stable non-pyrophoric solutions of lithium diorganoamides and processes for preparing them.
Due to their high Bronsted basicity and their low nucleophilicity, lithium diorganoamides, particularly lithium diisopropylamide (LDA), are used on a large scale for the synthetic preparation of pharmaceutical products and as specialty chemicals. Lithium diorganoamides have only a low solubility in hydrocarbon solvents and usually in such solvents an irreversible precipitation of oligomeric or polymeric crystallites of lithium diorganoamide occurs. For this reason, solutions of lithium diorganoamides in hydrocarbons are not commercially available. Whereas lithium diorganoamides are very soluble in ethers, such solutions are not conventional, because they decompose very quickly at room temperature. For this reason numerous users prepare lithium diorganoamides in the required amounts immediately before they are used. Their preparation is usually carried out by reaction of n-butyl lithium with diisopropylamine in cold tetrahydrofuran. But that preparation involves safety risks for those users who are not skilled in the handling the highly reactive organolithium compounds. Besides, the synthesis of lithium diisopropylamide by the reaction of lithium metal with diisopropylamine in diethyl ether in the presence of styrene or isobutene as electron acceptors is known from M. T. Reetz and W. F. Maier, Liebigs' Annalen der Chemie, 10, pp. 1471 to 1473 (1980).
The Patent document W0-86/03 744 discloses stable non-pyrophoric solutions of lithium diisopropylamide in a hydrocarbon solvent. These solutions contain a limited amount of tetrahydrofuran. The main disadvantage of lithium diisopropylamide in ethereal solutions or in hydrocarbon solutions which contain tetrahydrofuran in a limited amount of &lt;1.0 mole per mole of LDA is their limited stability. Solutions of LDA with an equimolar content of THF at maximum exhibit a significant loss of activity (25 to 50%) when stored at 30.degree. to 40.degree. C. for 30 days. On the other hand, only a small loss of activity has been observed during that time at temperatures from 0.degree. to 10.degree. C. But at storage temperatures &lt;0.degree. C. crystallization occurs in more than 2.0-molar solutions of LDA and THF.
German Patent Application DE-A-39 05 857 discloses bimetal diorganoamide compositions, particularly Li-Mg-bis-diorganoamides, in liquid hydrocarbon solvents. These compositions are solutions which have a higher thermal stability and better stability against precipitation than the pure lithium diorganoamide solutions.
Due to the presence of a second metal and/or a metal salt, purifying operations are required after one of these compositions has been used for a synthesis. Besides, the stability of these compositions over a comparatively long time should be improved because of the necessary presence of Lewis bases, such as THF, methyl THF, dimethyl ether, diethyl ether, dibutyl ether or tertiary amines, such as trimethylamine, triethylamine or tetramethylethylenediamine.